1. Field of the Invention
Embodiments of the invention generally relate to electronics, and in particular, to a digital data communication receiver using a Mueller-Muller based timing error detector.
2. Description of the Related Art
FIG. 1 illustrates a conventional Mueller-Miller timing error detector (MM TED). The illustrated MM TED 100 includes a slicer 102, delay elements 104, 106, multipliers 108, 110, and a subtractor 112. The slicer 102 converts soft symbol samples into hard symbol samples. The soft symbol samples can be generated by an analog-to-digital converter and has multiple bits. The hard symbol samples can be, for example, binary symbols, but can also have more than one bit depending on the system. The MM TED 100 is a well-known technique for extracting timing information in baud-rate sampled systems. See, for example Mueller, Kurt H., and Muller, Markus, “Timing Recovery in Digital Synchronous Data Receivers”, IEEE Transactions on Communications, Vol. COM-24, No. 5, May 1976, pp 516-531. The equation defining operation is given in Eq. 1.εMM=yn·ŷn−1−ŷn·yn−1  Eq. 1
In Eq. 1, “y hat” or ŷ represents the determined symbol and has an assumed value of +/−1. The first term of the equation represents an estimate of the Post-Cursor inter-symbol interference (ISI), while the second term represents an estimate of the Pre-Cursor ISI. The MM TED 100 based timing loop achieves timing by balancing the pre-cursor ISI with post-cursor ISI. As with typical digital timing recovery schemes, the error signal output of the MM TED 100 is filtered and fed back to control the time alignment of the analog-to-digital converter (ADC) sample clock signal.
Variations of the classic MM TED exist, such as in Mike Harwood, et al., “A 12.5 Gb/s SerDes in 65 nm CMOS Using a Baud-Rate ADC with Digital Receiver Equalization and Clock Recovery”, IEEE ISSCC 2007, pp. 436, in which only a pre-cursor ISI estimate is included. In this case, timing is driven (usually advanced ahead of the peak of the impulse response) until the pre-cursor ISI is reduced to zero.
Eq. 1 can be re-written in a slightly different form as shown in Eq. 2, without loss of functionality.εMM=yn·ŷn−1−yn·ŷn+1  Eq. 2
Eq. 2 simplifies as shown in Eq. 3.εMM=yn·(ŷn−1−ŷn+1)  Eq. 3
From inspection of Eq. 3, it can be observed that the timing error εMM is a function of the current symbol sample yn and the determined value of the previous symbol ŷn−1 and the next symbol ŷn+. When the preceding and next determined values are the same, the error signal output εMM of the MM TED 100 is zero. When the preceding and next determined values are different, the error signal output εMM of the MM TED 100 is proportional to the value of the current sample yn, with the sign of the proportionality being dependent on the values of the preceding ŷn−1 and next ŷn+1 determined values.
The use of Mueller-Muller Timing Error Detectors is well known in the current art. For example, refer to U.S. Pat. No. 7,646,807 to Manickam, et al., and U.S. Pat. No. 7,564,866 to Agazzi, et al., the disclosures of each of which is incorporated by reference herein. In each of these, a standard Mueller-Muller circuit is used to drive timing recovery.
For demanding applications, a number of advances have been made in recognition of the inherent timing alignment issue with standard Mueller-Muller timing error detectors.
In U.S. Patent Application Publication No. 2010/0080282 by Zhong, et al., the disclosure of which is incorporated by reference herein, the timing offset problem of Mueller-Muller detectors is identified and the impact on Sinusoidal Jitter Tolerance (SJTol) is recognized. Improvements in SJTol performance are achieved by improving the timing alignment of the Mueller-Muller detector.
In U.S. Patent Application Publication No. 2009/0135894 by Huang, the disclosure of which is incorporated by reference herein, the problem of timing alignment of the Mueller-Muller detector and an additional problem of elimination of ISI by a preceding equalizer are recognized. Huang addresses an absence of ISI, detection of which is exploited by a Mueller-Muller detector.
In U.S. Pat. No. 7,489,749 to Liu, the disclosure of which is incorporated by reference herein, the problem of timing alignment of the Mueller-Muller detector and the issue of initial convergence of digital equalizers used in the receiver are recognized. The Mueller-Muller uses detected symbols, and Liu addresses problems with initial startup for channels with heavy ISI.
In Linn, Yair, “Two New Decision Directed M-PSK Timing Error Detectors”, Proc. 18th Canadian Conference on Electrical and Computer Engineering (CCECE'05), May 1-4, 2005, pp. 1759-1766, the issue of MM TED gain variation with signal amplitude was identified and a normalization solution provided.